1. Field of the Invention
This invention relates to the field of aircraft flight control systems, and in particular to a method and system for yaw control augmentation.
2. Description of Related Art
In conventional high speed jet aircraft, the ability to provide flight attitude control at relatively low speeds and/or high altitudes is inherently limited because the ability to generate large control moments depends on the magnitude of pressure differentials generated by movement of air past the control surfaces which in turn depends on the speed and density of air which passes the surfaces during flight. A discussion of the factors which govern the design of control surfaces in high-speed jets is found in the article entitled "Aerodynamic Wave Drag" by Whitcomb, in McGraw-Hill Encyclopedia of Science and Technology, 6th Ed., McGraw-Hill Book Company, 1987.
While it is possible to increase the magnitude of control moments generated at low air speeds and high altitudes by suitable design of conventional external control surfaces, such designs necessarily increase the aerodynamic drag caused by those surfaces. Such increased drag is unacceptable in supersonic and near-supersonic aircraft.
Various approaches have been attempted to maximize control while minimizing drag, but the most common approach has been to simply sacrifice aircraft maneuverability and stability in favor of speed. Such compromises have severely limited aircraft performance in a number of areas. In particular, in the case of military fighter aircraft, speed and maneuverability are both essential. Even incremental increases in speed and maneuverability will result in significant increases in aerial combat survivability.
A more sophisticated approach has been to include mechanisms for altering the flight control surfaces according to speed. For example, at high Mach numbers, the wings of the jet may be "swept back" or retracted. This approach suffers from greatly increased complexity, resulting in higher cost, an increase in the probability of failure, and an increase in the weight and thrust requirements of the aircraft. Consequently, while several widely used fighters and other high speed aircraft have adopted this approach, many of the latest high speed aircraft designs continue to employ fixed external control surfaces, despite the above-noted disadvantages.
An alternate proposal has been to use the increased air speed at the jet engine air outlet for flight attitude control augmentation. However, control surfaces placed in the engine exhaust stream suffer from having to withstand the high engine discharge temperature and pressures.